This application claims priority under 35 USC 119(a) to Patent Application No. 2004-185351 filed in Japan on 23 Jun. 2004, the content of which is hereby incorporated herein by reference in its entirety.
The present invention relates to an electrode connection structure for use between/among circuit boards and to an electrode connection structure for use between/among inkjet head(s), semiconductor device(s), liquid crystal display panel(s), MEMS (microelectromechanical system) device(s), and/or the like and circuit board(s) such as flexible wiring board(s).
An electrode connection structure for use between an inkjet head and a flexible wiring board is proposed at Japanese Patent Application Publication Kokai No. 2002-127422 and elsewhere.
Referring to FIG. 9 (a) through (b), a conventional inkjet head will be described in general terms.
An ink chamber comprising a plurality of grooves is formed by lamination of actuator member 100, which is such that piezoelectric material subjected to poling in the thickness direction is laminated together so as to cause polarization directions to be in mutual opposition and which is such that a plurality of grooves are formed in the piezoelectric material; cover member 110, in which ink supply port 111 and common ink chamber 112 are formed; and nozzle plate 120, having nozzle hole 121. Note that the location at which nozzle plate 120 is arranged is taken to be the front end of the ink chamber, and the location opposite to this front end is taken to be the back end of the ink chamber.
The ink chambers are divided by means of partitions, ink chamber electrodes 101 for application of electric field(s) being formed on these partitions.
Formed at the back end of the ink chamber is R-shaped region 102, which is fabricated so as to be R-shaped; and also flat region 103, which serves as electrode lead region for connection with external circuitry. Furthermore, connection electrode 132 of flexible wiring board 130, on which is mounted drive IC (integrated circuit) 131, and electrode 104, which is formed over flat region 103 and which is for connection to inkjet head external circuitry, are electrically and mechanically connected by way of intervening ACF 15 (anisotropic electrically conductive film) 140, in which electrically conductive particles 142 are dispersed throughout epoxy-type resin binder 141.
Here, at the electrode connection region shown at FIG. 9 (b), connection electrode 132 of flexible wiring board 130 protrudes from the board substrate, made up of polyimide, by an amount corresponding to thickness T7 of connection electrode 132; moreover, inkjet head electrode 104 for connection to external circuitry protrudes from the inkjet head substrate, made up of piezoelectric material, by an amount corresponding to thickness T8 of inkjet head electrode 104 for connection to external circuitry. Accordingly, with a conventional electrode connection structure employing ACF 140 or other such adhesive, the thickness of ACF 140 intervening between adjacent electrodes will be approximately (T7+T8); or more accurately, this thickness will be the aforementioned (T7+T8) plus the thickness of the electrically conductive particles in the ACF when in its connected state.
By applying electric potentials of opposite phase to electrodes that face each other by way of intervening partitions at electrodes formed on partitions which form ink chambers, an inkjet head module formed as described above can be driven in shear mode. That is, partitions of ink chambers at lamination boundaries of partitions of ink chambers laminated so as to cause polarization direction to be oriented symmetrically in the thickness direction undergo deformation in v-like fashion; and by utilizing the change in volume within the ink chamber and the change in ink pressure within the ink chamber that occurs in accompaniment thereto, ink droplet(s) can be jetted from small nozzle(s) arranged at the front end of the ink chamber.
Referring to FIGS. 10 through 12, a method by which electrode(s) within ink chamber(s) in a conventional inkjet head are made to extend to the flat region will next be described.
Dry film resist 150 is laminated onto the main surface of the actuator member, photolithography is employed to create resist openings 151 only at portions of flat region 103 (see FIG. 12) at which electrodes are to be formed, and expose and develop operations are carried out. Dicing blade 160 of a dicer is then made to move in the direction indicated by arrow D101 (see FIG. 10) so as to half-dice the piezoelectric material, as a result of which the groove that will later become the ink chamber is formed, and dicing blade 160 is raised to form R-shaped region 102 at the back end of the ink chamber in correspondence to the diameter of the dicing blade. At this time, R-shaped region 102 is made to reach resist opening 151 in dry film resist 150 at flat region 103. After the ink chamber array is formed in this fashion, sputtering and/or plating technique(s) are employed to form metal film(s) comprising Al (aluminum), Cu (copper), Ni (nickel), and/or other such metal electrode material(s) within the ink chambers to form electrodes. Furthermore, in similar fashion, metal film(s) are also formed, forming electrodes, at resist openings 151 in dry film resist 150 at flat region 103 and R-shaped region 102 at the back end of the ink chamber to form inkjet head external circuitry connection electrodes 104 for connection to external circuitry.
Referring to FIG. 9, a conventional method for connecting inkjet head electrode(s) for connection to the exterior and flexible wiring board(s) will next be described.
Following alignment of inkjet head electrode 104 for connection to external circuitry, which is formed over flat region 103 of the aforementioned inkjet head, and connection electrode 132 of flexible wiring board 130, to which ACF 140 has previously been attached in preliminary fashion, a heating tool, not shown, is used to apply heat and pressure to electrically and mechanically connect inkjet head electrode 104 for connection to external circuitry and connection electrode 132 of flexible wiring board 130, completing the ACF connection operation.
Furthermore, in addition to connection between an inkjet head and flexible wiring board as has been described above, patent references (e.g., Japanese Patent Application Publication Kokai No. H06-23996 (1994), Japanese Patent Application Publication Kokai No. H10-44418 (1998), Japanese Patent Application Publication Kokai No. H10-100403 (1998),Japanese Patent Application Publication Kokai No. 2000-127404, and Japanese Patent Application Publication Kokai No. 2002-127422) have proposed connection between flexible wiring board and rigid printed wiring board, and connection between semiconductor device and flexible wiring board, but just as was the case for conventional connection structures between inkjet heads and flexible wiring boards, these are such that the thickness of ACF or other such adhesive at the electrode connection region is the same as, or is the same as or more than, the thickness of the electrode protruding from the board substrate.
A variety of problems such as the following can arise when a conventional circuit board connection structure as has been described above is employed to connect device(s) to circuit board(s) or is employed to connect circuit board(s) to circuit board(s).
With a conventional electrode connection structure between inkjet head(s) and flexible wiring board(s) employing adhesive(s), the thickness of the ACF or other such adhesive between the boards may be greater than or equal to the thickness of the two electrodes being connected. Adhesive typically having a high coefficient of linear expansion (60 ppm/° C. to 150 ppm/° C.), because a rise in temperature at the electrode connection region will cause adhesive at the electrode connection region to expand by a large amount, with thermal stresses acting in a direction such as will tend to spread the electrodes apart, there has been the problem of occurrence of electrical continuity failures. Furthermore, there has been the problem that the high magnitude of the strain experienced by adhesive with change in temperature causes adhesive force to deteriorate, and there has been the problem of early failure during thermal cycling reliability testing.
Furthermore, just as was the case for the inkjet head electrode connection structure described above, conventional electrode connection structures between circuit boards, e.g., such as those between flexible wiring board(s) and rigid wiring board(s), have the problems of electrical continuity failures at elevated temperature and early deterioration in reliability with thermal cycling.
The present invention was conceived in light of such state of affairs, it being an object thereof to provide a circuit board electrode connection structure having high reliability as a result of prevention of early deterioration in reliability with thermal cycling due to reduction in magnitude of strain due to thermal stress and/or prevention of occurrence of electrical continuity failures.